Technical Abstract:
Lowering soil pH is an important strategy to enhance phytoextraction. The hypothesis of this research is that elevated heavy metals at low pH have negative impacts on the soil microbial ecosystem, but these negative impacts can be alleviated following an increase in the soil pH once phytoextraction is complete. Two soils with high and low Cd contaminations were each adjusted to five different pH levels. Thlaspi caerulescens was grown for 6 months, and both the rhizosphere and bulk soil biological activities were tested after harvest. Soil basal respiration, nitrification potential, alkaline phosphatase, and arylsulfatase activities were significantly lowered by decreasing soil pH. While acid phosphatase activity was initially increased with decreasing pH, later, activity was reduced. The most probable number of rhizobia also found that low soil pH had fewer numbers of rhizobia present in the soil. Similarly, at the low soil pH, the number of bacteria, actinomycetes tended to decrease, while fungi tended to increase. Most negative effects observed in the bulk soil were alleviated by the rhizosphere influence. Rhizosphere soil pH was usually 0.2-0.8 unit higher than that of the bulk soil. Whether the higher biological activities in rhizosphere soil were caused by the higher pH and/or some other reasons, and the reason that the rhizosphere soil had higher pH levels require further research.